llile@SALTONUSA.COM wrote:
>
> I am using an external signal to shut down a PIC processor via MCLR.  I
> have a circuit that can generate a failure condition which will result in
> MCLR dropping to zero, shutting off my chip and resulting in a fail-safe
> condition.    I am powering MCLR from a circuit that has about 25V half
> wave AC, so I need to filter it, voltage divide it, and then clamp it.
>
> I'd like to clamp the voltage going into MCLR so that it doesn't exceed
> 5V.  Pulling out my trusty 1n5231B zener, I find that you have to crank
> the zener current up to 10-20 mA to get this zener to actually "knee" near
> 5V.  I first tried a zener current of 0.2 mA and got a pitiful soft zener
> knee of about 4.8 volts, not enough to reliably hold the PIC out of reset.
>   The tiniest spike would change the voltage a lot.


Use the resistor and zener you have, and add a
small capacitor across the zener. Unless you need
to change MCLR voltage rapidly that should fix
your spike issues and 0.2mA should be plenty. :o)
-Roman

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